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<p>Hi all,</p>
<p>I agree that changing time representation (rendering) is not a
transform in the sense of the model.</p>
<p>But changing the frame (time scale + refpos + direction) would
be.</p>
<p>The discussion below ios about going from a "World" TimeFrame to
another one.</p>
<p>But I see at least another use case : "movies" encoded as FITS
Cube where axis 3 is representing time</p>
<p>These kind of things exist in solar physics as far as I know.</p>
<p>In that case a Transform FROM pixels in axis 3 to a World
TimeFrame could be needed.</p>
<p>Cheers</p>
<p>François<br>
</p>
<div class="moz-cite-prefix">Le 03/04/2020 à 21:10, Arnold Rots a
écrit :<br>
</div>
<blockquote type="cite"
cite="mid:CAJXToE-yfVP81QtsrLvFUqJ4B3yX1ojN-pO+r+-GSQWVKQK6vw@mail.gmail.com">
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<div>Yes, the spacecraft-to-earth case is a valid transform.</div>
<div>I only responded to the line I quoted.</div>
<div><br>
</div>
<div>That said, there is more to it (and I realize I will be
repeating comments I have made before).</div>
<div>Defining a time stamp using the (M)JD/UTC combination is a
bad (dangerous) idea; UTC measures an angle, it is not a
chronometer.<br>
</div>
<div>Earlier Transform designs focussed on spatial coordinates,
emulating the FITS WCS development in that area (WCS Paper
II).<br>
</div>
<div>Other simple coordinate types were automatically included
(because it was easy to do).</div>
<div><br>
</div>
<div>Time is a different beast.</div>
<div>There is the transformation from one time scale to another,
with or without a change in reference position.</div>
<div>But, if done properly, the relative motion of these
reference positions should be taken into account.</div>
<div>And then there is the option of applying a pathlength
correction, which is relevant when measuring photon arrival
times.</div>
<div>One may need to take gravitational potentials along the way
into account.</div>
<div>But some of this may not be relevant for transforming
spacecraft time stamps, since many spacecraft missions (at
least the ones I am aware of) artificially synchronize their
spacecraft clocks with TAI on the surface of the earth,
thereby zeroing out all relativistic effects (gravitational
potential and acceleration) that the spacecraft clock is
subject to. In a way, if you want to be pedantic, it means
that effectively fundamental physics constants vary on board.</div>
<div>All this to say that time transformations are a can of
worms that is best left to a specialized library. Leave it to
David Berry.<br>
</div>
<div><br>
</div>
<div>As to Mark's comment regarding the timing of FITS WCS Paper
IV, if one looks closely, one may realize that that paper is
based on HEA time conventions combined with the experience
gained in the development of STC1 - which, in turn, had some
roots in the ISAIA program. And so, the design of the temporal
domain that I introduced in STC2 pretty much continued what
was contained in Paper IV. I admit that I have no idea how
much of that is left at this point.</div>
<div><br>
</div>
<div>Cheers,</div>
<div><br>
</div>
<div> - Arnold<br>
</div>
<div>
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<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Fri, Apr 3, 2020 at 11:51
AM Laurent MICHEL <<a
href="mailto:laurent.michel@astro.unistra.fr"
moz-do-not-send="true">laurent.michel@astro.unistra.fr</a>>
wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Arnold
,<br>
<br>
In my example, I was talking (a bit ahead) about transforming
spacecraft <br>
time to earth time. The might be real a transform (change of
scale/ref) <br>
isn't it?<br>
<br>
LM<br>
<br>
Le 03/04/2020 à 16:33, Arnold Rots a écrit :<br>
>>>> You mean like DATE => MJD? GMT - PST?<br>
> > yes I do<br>
> <br>
> With all due respect, these are not transformations.<br>
> They are properties (renderings, if you like) of the pure
coordinate <br>
> value, without needing any coordinate frame information.<br>
> If you want to transform a pixel coordinate to, say, a
ICRS equatorial <br>
> coordinate, that is a true transformation, requiring
information about <br>
> the relation between the two coordinate frames involved.<br>
> On the other hand, if I have a time stamp value that is
kept, for <br>
> instance, in JD, I can ask it to be rendered in MJD or
ISO-8601. No <br>
> information external to that JD value is required, not
even its time scale.<br>
> True time transformations would be required for changing
the time scale <br>
> of a particular time instance, or its time frame's
reference position.<br>
> But that is a much more involved subject.<br>
> <br>
> Cheers,<br>
> <br>
> - Arnold<br>
> <br>
> Arnold H Rots<br>
> <br>
> Research Associate<br>
> <br>
> SAO/HEAD<br>
> <br>
> Center for Astrophysics | Harvard & Smithsonian<br>
> <br>
> <br>
> Email: <a href="mailto:arots@cfa.harvard.edu"
target="_blank" moz-do-not-send="true">arots@cfa.harvard.edu</a>
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target="_blank" moz-do-not-send="true">arots@cfa.harvard.edu</a>><br>
> <br>
> Office: +1 617 496 7701 | Cell: +1 617 721 6756<br>
> <br>
> 60 Garden Street | MS 69 | Cambridge, MA 02138 | USA<br>
> <br>
> <br>
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> On Fri, Apr 3, 2020 at 5:04 AM Laurent MICHEL <br>
> <<a href="mailto:laurent.michel@astro.unistra.fr"
target="_blank" moz-do-not-send="true">laurent.michel@astro.unistra.fr</a>
<br>
> <mailto:<a
href="mailto:laurent.michel@astro.unistra.fr"
target="_blank" moz-do-not-send="true">laurent.michel@astro.unistra.fr</a>>>
wrote:<br>
> <br>
> Hello,<br>
> <br>
> FITS paper II and III are listed in the draft (page
6) but not FITS<br>
> paper IV (time representation).<br>
> It's a bit odd.<br>
> To me, time transformations are symmetric with e.g.
spectral coordinate<br>
> transformations, even if they are less used in FITS
files.<br>
> <br>
> Few comment below:<br>
> <br>
> Le 02/04/2020 à 17:35, David Berry a écrit :<br>
> > On Thu, 2 Apr 2020 at 15:55, CresitelloDittmar,
Mark<br>
> > <<a href="mailto:mdittmar@cfa.harvard.edu"
target="_blank" moz-do-not-send="true">mdittmar@cfa.harvard.edu</a>
<mailto:<a href="mailto:mdittmar@cfa.harvard.edu"
target="_blank" moz-do-not-send="true">mdittmar@cfa.harvard.edu</a>>>
wrote:<br>
> >><br>
> >><br>
> >> You mean like DATE => MJD? GMT - PST?<br>
> yes I do<br>
> <br>
> >> I think these are in the same level as<br>
> ENERGY-FREQUENCY-WAVELENGTH, which are really
standardized<br>
> transforms which are basically considered different
forms of the<br>
> same value. Which is why they are in the Coords
model as different<br>
> 'flavors' of Time coordinate rather than having a
single time<br>
> coordinate and using transforms to convert.<br>
> You need a transform to go from a flavor to another.<br>
> If I want to use TRANSF to model e.g. a ground
segment processing, I<br>
> might have a step converting spacecraft time to earth
time.<br>
> This is very similar with converting pixels to RA/DEC
by using attitude<br>
> data among other things or converting spectrometer
channels to kEv.<br>
> <br>
> ><br>
> > I think it depends to what extent we want to
limit the use of the<br>
> > Transform model. Being able to describe the
mapping between different<br>
> > coordinate systems in a single physical domain
seems like a<br>
> > reasonable goal for a Transform model. For
instance, if I have an<br>
> > image in which the WCS gives the (ra,dec) as a
function of pixel<br>
> > position, I may want to create a copy of that
image that gives the<br>
> > (l,b) of each pixel instead of (ra,dec). To do
this I would need to<br>
> > modify the WCS by tagging on a Mapping to
convert (ra,dec) to (l,b).<br>
> ><br>
> > Original WCS: (pixel) -- mapping 1 ->
(ra,dec)<br>
> > New WCS: (pixel) -- mapping 1 -> -- mapping
2 -> (l,b )<br>
> ><br>
> > i.e. "mapping_1" is the pixel to (ra,dec)
mapping from the original<br>
> > image, and "mapping 2" is the (ra,dec) ->
(l,b) mapping.<br>
> <br>
> Agree, but the time is one of these WCS domains
(paper II III and IV)<br>
> isn't it?<br>
> <br>
> <br>
> > Being able to modify a WCS so that it
represents a different physical<br>
> > coordinate system seems like a reasonable
use-case. But maybe one<br>
> that<br>
> > can be deferred to a later date. The beauty of
the sort of system we<br>
> > are creating is that it is is easy to extend
it. What matters most is<br>
> > that we get the right definition of mappings,
transforms, operations,<br>
> > axes and so on.<br>
> Correct me if I'm wrong, there is no need to modify
WCS to deal with<br>
> time coordinates.<br>
> <br>
> I think that time transformations should be part of
the model, also<br>
> because Time Data still are an high VO priority.<br>
> If it is not, e.g. because we urge to go in PR, this
must be justified<br>
> in section 2.<br>
> <br>
> Cheers<br>
> Laurent<br>
> <br>
> -- <br>
> ---- Laurent MICHEL Tel (33 0) 3 68 85
24 37<br>
> Observatoire de Strasbourg Fax (33 0) 3 68
85 24 32<br>
> 11 Rue de l'Universite Mail<br>
> <a href="mailto:laurent.michel@astro.unistra.fr"
target="_blank" moz-do-not-send="true">laurent.michel@astro.unistra.fr</a>
<mailto:<a href="mailto:laurent.michel@astro.unistra.fr"
target="_blank" moz-do-not-send="true">laurent.michel@astro.unistra.fr</a>><br>
> 67000 Strasbourg (France) Web <a
href="http://astro.u-strasbg.fr/~michel" rel="noreferrer"
target="_blank" moz-do-not-send="true">http://astro.u-strasbg.fr/~michel</a><br>
> ---<br>
> <br>
<br>
-- <br>
---- Laurent MICHEL Tel (33 0) 3 68 85 24 37<br>
Observatoire de Strasbourg Fax (33 0) 3 68 85 24 32<br>
11 Rue de l'Universite Mail <a
href="mailto:laurent.michel@astro.unistra.fr"
target="_blank" moz-do-not-send="true">laurent.michel@astro.unistra.fr</a><br>
67000 Strasbourg (France) Web <a
href="http://astro.u-strasbg.fr/~michel" rel="noreferrer"
target="_blank" moz-do-not-send="true">http://astro.u-strasbg.fr/~michel</a><br>
---<br>
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